Optical filters are key components for optical communication applications such as wavelength-division multiplexing (WDM), signal processing, and dynamic bandwidth allocation. These key devices are now desirable in integrated photonic systems, especially on silicon-on-insulator (SOI) wafers, to achieve the same functions as traditional discrete optical components such as diffractive grating spectrometers. Silicon photonics allows for complementary metal-oxide-semiconductor (CMOS) compatible mass fabrication for low cost, high yield, and large-scale on-chip integration.
Future high-capacity transmission systems, e.g., applying the super-channel technique, will require a dynamic channel bandwidth allocation over a few hundred gigahertzes. However, large bandwidth tunability is currently only available in bulky bench-top systems. Existing integrated tunable filters, e.g., using microring resonators and Mach-Zehnder interferometers (MZIs), have relatively small tunable bandwidth (less than 200 GHz). Also, their free spectral ranges (FSRs) are small, typically less than 10 nm. There always remains room for improvement.